It's kind of a pain.
The bezel is aluminum. You can just pry it off at the bottom with a screwdriver.
Putting a new one on takes some work. You just reassemble, but they were originally stamped on, but to replace, you have to peen over the edge, while applying pressure so it's tight.
If you can save the original lens, do it. They polish out pretty well. The original lenses were domed slightly and the replacement ones are flat. They work, but look a little different.
I might also recommend Speedo King. They'll check and fix anything broken.
http://speedoking.com/
Speedo King rebuild